Stop ignoring global leaks

The `goog.dom.ViewportSizeMonitor` (used in map.js) creates a global leak by setting `goog.UID_PROPERTY_` on the monitored window.  In order to test that we don't have other global leaks, we preemptively set the property so Mocha can compare the global before and after our tests.
Merge pull request #422 from tschaub/stop-ignoring-global-leaks

Stop ignoring global leaks